Munitions such as missiles and rockets are often packaged in canisters for environmental protection, ease of handling, and efficient and long term storage. These canisters are typically round or square tubes that contain the munition, the munition launch hardware such as rails and/or sabots, and electronics for initiating the launch of the munition. In order to provide full environmental protection of the munition and these components, covers must be installed to seal the canister and thereby contain the munition and components in a completely closed environment. The cover must be able to provide environmental protection to the munition under a variety of extreme conditions, e.g., high altitude, variations in humidity, low and high temperatures, debris impact from transportation, and exposure to water. Upon initiation of a munition launch, the canister cover must be easily and quickly removed to allow unimpaired exit of the munition from the canister.
There are several types of canister covers currently utilized. For example, a flexible membrane can be stretched over a frame which is permanently attached to the canister. Upon launch initiation, the munition tears through the membrane. Disadvantages of using a membrane as a canister cover include limited durability, limited environmental protection, and inefficient storage due to the space required for the frame used to attach the membrane to the canister.
Another currently used canister cover is a hard cover made of plastic or metal, with the cover being adhesively bonded or attached to the canister. Upon launch initiation, the cover is blown off the canister by either launch pressure or the forward motion of the munition. The hard cover may be partially cut or scored to allow the cover to break into a predetermined number or size of pieces, such as into quarters or halves. Disadvantages of such a cover include the high launch pressures required to break the hard cover or to break the bond between the cover and the canister, inconsistent breaking of the bond between the cover and the canister, damage to the munition caused by impact with the cover, and damage to launch equipment caused by cover fragments.
Foamed plastic covers, comprised of a solid layer of plastic foam, are also currently utilized to cover munition canisters. Upon initiation of munition launch, the munition strikes the foamed plastic cover which then breaks into multiple pieces. Foamed plastic covers, however, provide a poor environmental seal because they are comprised of a porous material that has limited resistance to air or moisture. Furthermore, because the foamed plastic is hard and brittle, it is difficult to attach to a canister without breaking or cracking the foamed plastic.
Canister covers fabricated from glass have also been considered. Glass is low cost, lightweight and provides excellent environmental protection. Glass can be heat treated (tempered) in such a manner that it becomes frangible, i.e. a very small point load will completely shatter a large plate of the glass. Variations in the heat treatment allow the size of the shattered pieces to be predetermined and controlled. Tempered glass is often used in applications such as automobile windows or glass doors. One disadvantage of using tempered glass for canister covers is that such a cover has low overall durability, primarily due to the ease with which the glass can be shattered. Another disadvantage of tempered glass is that, even with the ability to control fragment size through heat treatment, the size of the fragments cannot be made big enough so that they do not interfere with the forward movement of the munition from the canister and do not damage the surfaces of the munition. Small fragments interfere with the egress of the munition by becoming wedged between the outer surfaces of the munition and the inner surfaces of the canister; small fragments can also scratch the outer surfaces of the munition, thereby changing the aerodynamics of the munition. In contrast, large fragments would be blown away from the canister due to pressure from the launch or from impact by the munition. It would therefore be desirable to have a glass canister cover having both high durability and large fragment size upon munition launch.